[Bug 232835] [gmirror] gmirror fails to recover from degraded mirror sets in some circumstances (2/n)

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=232835

            Bug ID: 232835
           Summary: [gmirror] gmirror fails to recover from degraded
                    mirror sets in some circumstances (2/n)
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: cem at freebsd.org
                CC: geom at FreeBSD.org, markj at FreeBSD.org
        Depends on: 232671
            Blocks: 232683, 232684

This is related to bug 232671 , but not identical.

Here is the example scenario:

1. I start with a GMIRROR with three ACTIVE disks
   (sc_ndisks = md_all = 3)

2. I essentially disconnect one of the disks

3. The remaining mirrors lower md_all to 2 and the syncid /
   generation gets bumped

4. I reboot, and the removed disk reappears

5. Geom tastes the stale removed disk first, and populates
   sc_ndisks from its md_all (3)

6. The two valid mirrors are tasted afterwards and the gmirror
   rejects both as having invalid metadata (md_all=2), despite
   their having a higher generation / sync id

The problem is basically that gmirror doesn't "upgrade" its metadata
to the newest valid mirrorset it finds -- it just sticks with whatever
it found first.

I think the solution is being a bit clever about detecting the latest
mirror generation while a gmirror is still in the STARTING state;
and also perhaps a little more clever about when we transition from
STARTING to RUNNING (at which point a newer generation mirror showing
up means we have corruption).


Referenced Bugs:

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=232671
[Bug 232671] [gmirror] gmirror fails to recover from degraded mirror sets in
some circumstances
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=232683
[Bug 232683] [gmirror] gmirror could provide much better administrative
introspection into decision-making processes
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=232684
[Bug 232684] [gmirror] gmirror overly aggressive provider destruction
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